SEMObjectiveProcedureMatSE 471Laboratory 4SEMObjectiveThe objective of this laboratory is to learn the fundamentals of SEM operation.Background The optical microscope is a powerful tool for examining microstructures, but often the magnification isn't high enough. The optical microscope is limited to a resolution of about 0.5 m (or, equivalently, a maximum useful magnification of about 1000x) because the wavelength of light is itself around 0.5 m. To examine objects in finer detail requires radiation with a smaller wavelength, such as that produced using electrons. Microscopes which use electrons to provide images of samples are termed "electron microscopes." Electron microscopes apply a large voltage to a small filament (usually a tungsten wire), causing the filament to release electrons. The wavelength associated with the emitted electrons is dictated by the conservation of energy: the potential energy supplied by the applied voltage is converted to kinetic energy once the electron escapes. With an applied voltage (usually called an accelerating voltage) of about 30kV, a scanning electron microscope can produce electrons with a wavelength of 0.071 A, as shown by the following sequence of equations:P.E.= K.E. (1)(Potential energy supplied by accelerating voltage = Kinetic energy of electrons)Writing the expressions for potential and kinetic energy: qV0 = 21mv2(2)(q = electron charge, V0 = accelerating voltage, m = electron mass and v = electron velocity)=mvh (3)(This is the de Broglie relation, which relates an electron's wavelength to its momentum. h=Planck's constant)Substituting eq. (3) into eq (2):02mqVh=λ (4)The values of the constants in eq. (4) are: h = 6.626 x 10-34 Js; q=1.602 x 10-19C, andm = 9.11 x 10-31 kg. To make the units work out, V0 should be in volts. Then  will be in meters.Note that if the resolution of the SEM was determined only by the wavelength of the electrons, we should be able to see objects as small as individual atoms! Unfortunately, it is not the wavelength of the electrons, but rather the width of the electron beam which limits the resolution of the SEM. When the electron beam is focused on a particular spot on the sample, all the information from that spot is detected together. Features smaller than the spotsize cannot be resolved. Although resolution goes up as spot size goes down, generally, the highest useful magnification for a conventional SEM is about 20,000X, corresponding to a resolution of about 25 nm. Note that the word "scanning" in scanning electron microscope refers to the fact that the electron beam is rastered across the sample, so that the image seen is built up spot by spot and line by line on a monitor screen.In an SEM, the image does not usually result from the electrons which hit the sample, but from the electrons which are originally inside the sample, and which are ejected when the first electrons hit. These latter electrons are "secondary electrons", and the instrument has a specially built detector to capture these secondary electrons. Secondary electrons are low energy electrons, and can only escape from the first 50-100 Å of the sample surface. Consequently, the SEM provides an image of the sample surface only. This image differs considerably from an optical micrograph. First, the SEM provides a much greater depth of field than an optical microscope, which means that even samples with very rough surfaces can be completely in focus. Second, the contrast in an SEM is not dictated by surface topography, as in the optical microscope (where shadows falling from one surface object ontoanother provide the surface contrast). Instead, contrast in the SEM is determined by how many electrons are emitted by the particular region of the sample that the electrons hit. The number of electrons emitted is, in turn, determined mostly by the atomic weights of the elements present. Phases containing heavy elements tend to appear light, and phases containing light elements tend to look dark. This is particularly true when the SEM is operated in the "backscattered mode". (In the backscattered mode, the SEM is set to detect backscattered electrons rather than secondary electrons. Backscattered electrons are those that come from the original beam, but are scattered or reflected backwards.)In this first SEM lab, you will learn the fundamentals of the operation of the instrument. TheTA will provide a specimen to be examined. If the specimen is not electrically conductive, it must be coated with a layer of gold only a few angstroms thick to make it conductive. Otherwise, the electrons which hit it could not be conducted away; they accumulate, and the specimen "charges up". A concentrated charge like this looks like a bright white spot on the screen. If enough charge builds up, the specimen could "burn" (the specimen is "fried"). The coating does not significantly obscure the surface features of the specimen.ProcedureStep-by-step instructions for the operation of the SEM are given in Appendix 4.1. Each student will record three images the specimen, each showing a different